This invention relates to the transport of tape between reels, and, more particularly, to tension control of tape during direct transport between the reels without significant buffering of the tape.
A tape that is directly transported between a supply reel and a take-up reel requires precise control of the tape tension to provide smooth constant tape speed, and, if the tape is to be accelerated and decelerated, to provide smooth acceleration and deceleration of the tape, and to prevent damage to the tape. One example of a direct tape transport is a magnetic tape drive, in which a magnetic tape is transported along a tape path past a read/write head located in the tape path, such that the magnetic read/write head may read and/or write data on the magnetic tape, and the magnetic tape is stopped and started to read and/or write data at desired locations of the magnetic tape. Another example of a direct tape transport is a printing press in which a tape (paper web) is transported between reels past printing rolls under precise timing, and new rolls of paper web must be accelerated to speed smoothly without damage and without smudging at the printing rolls. Herein, the term xe2x80x9ctapexe2x80x9d is defined as comprising tape or web in any suitable elongate form; the term xe2x80x9creelxe2x80x9d is defined as comprising a reel or spool that is removable or permanent, and provides the spool or axis at which the tape is wound and/or unwound; and the terms xe2x80x9csupplyxe2x80x9d and xe2x80x9ctake-upxe2x80x9d reels refer to the two reels of a two reel system, typically respectively from which the tape originates and to which the tape is directed, but the tape may be wound and unwound with respect to either reel in a bi-directional fashion.
One or more electromechanical tape tension transducers can be mounted in the tape path to monitor tape tension, and the supply and/or take-up reel motors are controlled to provide a nominal tape tension and to tend to offset errors in tape tension. Examples of tape tension transducers comprise U.S. Pat. Nos. 5,282,586, 5,277,378, 5,039,027, 4,807,107, and 3,606,201, all of which illustrate the use of tension arms. U.S. Pat. No. 3,809,335 indicates that other types of sensors may alternatively be used, such as a pressure responsive air jet or bearing, or a load cell. Another example of a tape tension transducer is a direct sensing pressure transducer. However, a delicate balance is maintained in designing tape tension transducers to both be robust and reliable, yet also to be accurate throughout a wide range of frequencies of tape tension variation, without resonances. The tape tension transducers, by virtue of the mechanical aspects, have upper limits to the effective range of tension measurement, for example, defined by resonance of the mechanism. U.S. Pat. No. 4,400,745 estimates tension by summing the currents from both the supply reel and the take-up reel motors, divides the result by two, and equates the same to tape tension. In U.S. Pat. No. 3,913,866, a signal proportional to the angular velocity of the supply and take-up reels is generated at each reel and supplied to a torque device at the other reel. Japan JP6-349153 compares a frequency of a motor or reel to an expected frequency to correct the torque of a motor to obtain stable tape tension without providing a tension lever or arm. Such rough estimates of the tape tension are not sufficiently accurate for modern high speed tape motion.
A precise control of tape tension is illustrated by U.S. Pat. No. 4,015,799 which determines tape tension based on the differences in torque as applied to the supply and take-up reels. Lineal tape position and angular reel displacement for both reels are monitored to determine reel radii, tape inertia and velocity or position error. A motor current algorithm is utilized to generate the appropriate torque for each reel to drive the error to zero along a predetermined profile with negligible tape tension disturbances. U.S. Pat. No. 5,860,610 discusses defining the reel inertia similar to that of the ""799 patent, employing a conversion table, to control the torque of a motor, and a second embodiment in which the output of a pressure sensitive tension sensor is differentiated and combined with the output of a torque modulator. In a third embodiment, change in rotating speed of a supply reel is employed with the torque modulator in an attempt to suppress the change in speed.
Modern tapes are driven at high speeds and are subject to high acceleration and deceleration rates, and are more sensitive to changes in tape tension, for example, in that the speed variation at the tape head will result in data read or write errors, or smudging at a print roll. In the case of magnetic tape, modern tapes are thinner to allow more tape to be spooled on a reel at the same diameter, and thereby to allow a greater data storage capacity. Hence, such tapes are more sensitive to changes in tape tension, and may be subjected to damage, such as tape stretch.
In accordance with the present invention, tension control systems, methods, and tape transports provide dynamic tension control for tape transported along a tape path between a supply reel and a take-up reel, the supply reel driven by a supply reel motor, and the take-up reel driven by a take-up reel motor.
In one embodiment, a supply tachometer measures the rotational angular displacement of the supply reel, and a take-up tachometer measures the rotational angular displacement of the take-up reel. A tension control system controller:
upon accelerating the tape toward a target tape speed;
initially operates the supply reel motor and the take-up reel motor in static acceleration tension control;
measures the rotational angular displacement of at least one of the supply reel from the supply tachometer, and of the take-up reel from the take-up tachometer;
determines from the measured rotational angular displacement, the rotational angular velocity of at least one of the supply reel and the take-up reel;
determines from the rotational angular velocity, a linear speed for the tape;
if the linear speed of the tape is less than a predetermined range of the target tape speed, continues the static acceleration tension control;
if the linear speed of the tape is within the predetermined range of the target tape speed, operates the supply reel motor and the take-up reel motor in composite tension control employing static tension control and delta velocity control.
In another embodiment, a tape tension transducer is provided in the tape path for measuring the tension of the tape, and the controller operates the supply reel motor and the take-up reel motor in static acceleration tension control, additionally employing direct tension sensing of the tape tension transducer; and operates the supply reel motor and the take-up reel motor in composite tension control, additionally employing direct tension sensing of the tape tension transducer.
In a further embodiment, the controller operates the supply reel motor and the take-up reel motor in composite acceleration tension control towards the target tape speed and in composite steady state tension control while maintaining substantially the target tape speed.
In a still further embodiment, the controller, in initially operating the supply reel motor and the take-up reel motor in static acceleration tension control, in continuing the static acceleration tension control, and in operating the supply reel motor and the take-up reel motor in composite tension control, each comprises the controller operating the supply reel motor and the take-up reel motor employing the direct tension control for tension variation of frequencies less than a predetermined frequency. In one embodiment, the predetermined frequency is less than a resonant frequency of the tape tension transducer.
In another embodiment, the controller, in operating the supply reel motor and the take-up reel motor employing composite tension control, comprises the controller determining the rotational angular velocity of the supply reel and the take-up reel, and operating the supply reel motor and the take-up reel motor employing the composite tension control if the rotational angular velocity of the reels each exceeds a predetermined minimum rotational angular velocity related to a minimum sample rate of the determination of the rotational angular velocity.
In still another embodiment, the controller:
upon decelerating the tape from a steady state tape speed;
initially operates the supply reel motor and the take-up reel motor in composite deceleration tension control employing static tension control and delta velocity control;
measures the rotational angular displacement of at least one of the supply reel from the supply tachometer, and of the take-up reel from the take-up tachometer;
determines from the measured rotational angular displacement, the rotational angular velocity of at least one of the supply reel and the take-up reel;
determines from the rotational angular velocity, a linear speed for the tape;
if the linear speed of the tape is within a predetermined range of the steady state tape speed, continues the composite deceleration tension control;
if the linear speed of the tape is less than the predetermined range of the steady state tape speed, operates the supply reel motor and the take-up reel motor in static tension control.
In a still further embodiment, the controller operates the supply reel motor and the take-up reel motor in composite deceleration tension control, additionally employing direct tension sensing of a tape tension transducer; and operates the supply reel motor and the take-up reel motor in static tension control, additionally employing direct tension sensing of the tape tension transducer.
For a fuller understanding of the present invention, reference should be made to the following detailed description taken in conjunction with the accompanying drawings.